Jennifer Aniston & Justin Theroux Throw Star-Studded Holiday Party

There was no lack of excitement in Jennifer Aniston and Justin Theroux's household over the weekend.

The recently-engaged pair kicked off December with a tree-trimming party, which has been an annual tradition for Aniston, 43.

On Sunday, pals including, Demi Moore, Ellen DeGeneres, Portia de Rossi, Ben Stiller, Christine Taylor, Tobey Maguire, Jennifer Meyer and some of Aniston's employees came out to the couple's Beverly Hills rental house for a family-friendly fete.

"The party was for both adults and children," a source tells PEOPLE. "Jen loves having children around and she always has fun activities planned for them. While the adults mingled, the younger guests were able to color and play games."

Adds the source: "Jen always puts a lot of effort into planning her holiday party. It's a thank you party for her employees, and she always gives them thoughtful gifts. Jen always asks her celebrity friends for charity donations instead of personal gifts."

Picking out a Christmas tree is no light matter for Aniston, who selected a 10 ft. noble fir at Mr. Greentrees in Beverly Hills.

"It's a tradition for her," another source says. "She is very specific about the tree that she wants. For the second year in a row, she picked out a tree with [Justin]. She seemed very excited and happy about the tree. [Justin] was very sweet to her and let her take her time to find the perfect tree. She found a beautiful one that she had delivered."

As for the upbeat couple, they "are doing really well together," the source says. "Jen always seems so happy. Justin takes great care of her. ... They are both looking forward to a break. They had an intense fall with a lot of work."